Deleting SKNode on contact with SKPhysicsWorld Boundaries in Swift 3

I am looking to create a unique opening screen for an IOS app, and I would like to have a background of balls dropping to create visual interest. I am creating the balls in the didMove() function, and they are working fine, but only some of the time. I believe the frame is too large for the display, which is making the random X and Y coordinates sometimes occur outside of the screen.